A curious kobold crept along the river, sniffing the air. He smelled Dwarves nearby. He crept closer to the hill, where he found a rusty metal pipe covered with brush. A delicious scent wafted out, very mushroomy, with a hint of something sweet. All the kobolds had been eating were the bundles of grass wafers they brought from the homeland.
The kobold snuck around the hill. He spotted a kitten tied to a root, and the kitten spotted him. It mewed curiously. The kobold threw a rock and knocked it out. He crept onward, and saw a few other kittens tied up around the perimeter inside the old wall.
He sidled up to the old, scarred door. Bones littered the ground, both Dwarven and Elven, and plenty of animal bones as well. There were new footprints. And the lock on the door had been repaired.
Out came the kobold lockpicking tools: a few bent pieces of wire, a flat metal shimmy, a leather flask of oil for the hinges, and a thin knife. He worked the lock, hunched over it, and suddenly heard a noise behind him!
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The Human villagers strode up the hill, angry, bearing pitchforks and shovels, pickaxes and torches. There was quite a crowd. Dogs ran up and down the mob, and a few agitators kept them moving. They approached the old Dwarf Fortress, past the old wall, right up to the door. They started banging down the door with their picks.
Suddenly the door sprang outward! The foot-thick stone slab smashed into four of the villagers, trapping two underneath and stunning a couple others. Then the snap of a rope, and a great crash. From inside the entrance rolled an enormous stone gear, 3 feet thick and seven feet across. The mechanism rolled straight out, grinding down two villagers before striking a rock and spinning out of control. The spinning Felsite mechanism struck many villagers, landing beyond the fallen door, directly on the mob's main agitator. His hand was all that stuck out from under the gear.
And in their panic, the villagers dropped their tools, some their hats and shoes, in their haste to flee the deathtrap. Of the angry mob that came, only a few stragglers left.
A short time later, two Dwarves wandered out of the entrance. They admired the nice door lying on the ground. But the stone mechanism was what really impressed them. It was completely carved on the visible side, with images of Tentacle Demons striking down Elves. The Tentacle Demons were laughing. When they picked it up, and scraped off the villager under it, they found the other side to be carved with a rendition of an image of a Dwarf eating cheese.
They rolled the mechanism back into the dark passage, and left it lying against the wall for the mechanic to reset his trap. By unspoken agreement they decided that the cheese engraving should be on the wall-side.
They went back for the door, to replace it on its hinges. But while they were bringing in the gear, a quiet shape slipped in and blended into the darkness.